you dont need more than 512mb graphic card, there are 512mb end cards that are better than 1gig, just make sure its a good one and it support the game decently
mac is really nice, you can even use windows on it without much problems as far as i know
the only problem is the price as everyone know, but mac try hard to make all the hardware compatible which increase the price, and all that power in a pc size of a screen..... its just amazing :P
there is the "desktop" mac, i forgot its name, its same concept as the pc
http://www.apple.com/macpro/
if you are interested in it, a bit too expensive though and price doesnt include the screen unless you chose it from the options.
you dont really need best graphic cards for wow to begin with, as the game is cpu dependent more than new games are dependant on graphics card
